Generally, eyeglass lenses are imparted with various properties by forming various functional films on the lens substrate while achieving a desired refractive index by means of the lens substrate. Antireflective films imparting an antireflective property to the lens surface are widely employed as such functional films. Among these, vapor-deposited films of zirconium (Zr) oxide are inexpensive among the high refractive index layers constituting multilayer antireflective films, and are thus considered advantageous in terms of cost (for example, see Japanese Unexamined Patent Publication (KOKAI) No. 2009-193022, which is expressly incorporated herein by reference in its entirety). Vapor-deposited films of tantalum (Ta) oxide can also function as high refractive index layers in the same way as vapor-deposited films of zirconium oxide.
Eyeglass lenses are required to have good durability that does not deteriorate even when placed in various environments. For example, eyeglass lenses are sometimes worn in the bath, left in automobiles in summer, and worn by wearers who are active for extended periods outdoors. There is a need for them to maintain good quality without developing cracks even when exposed to such elevated temperatures. In eyeglass lenses having vapor-deposited films of zirconium oxide and tantalum oxide, cracks may develop in the vapor-deposited films at elevated temperatures, resulting in phenomena such as a drop in optical characteristics and decreased adhesion between layers constituting the multilayer antireflective film.
Eyeglass lenses are also required to have good resistance to scratching without developing scratches or cracks with the application of external forces. Eyeglass lenses are subjected to various external forces in the course of removing fingerprints, debris, and other grime adhering to the lens surface. The scratches and cracks that develop due to such external forces cause phenomena such as decreased optical characteristics in the eyeglass lens and diminished adhesion between layers constituting multilayer antireflective films.